1. Technical Field
The present invention relates generally to the disposal of hazardous waste that includes both vitrifiable and volatile components. Specifically, the present invention relates to the decomposition of such waste by the application of an extremely high temperature plasma arc such as the flame of a plasma torch.
2. Background Art
One known method of decomposing hazardous waste is thermal free radical pyrolysis. Hazardous waste materials such as PCB's and dioxin are disassociated into their elemental constituents when subjected to high temperatures such as temperatures in excess of 10,000.degree. F. provided by a plasma arc. Upon cooling under controlled conditions, the elemental constituents will recombine into elemental forms and simple, non-hazardous compounds.
In many cases the pyrolysis of hazardous waste materials is complicated by the presence of a vitrifiable solid component such as silica. Although gasses and vapor are readily disassociated into their elemental constituents when passed through the gap between the electrodes of a non-transferred plasma torch, the vitrifiable solids such as silica, aside from being highly abrasive, tend to melt and clump together, forming an insulating coating on the electrode surfaces. Therefore it is impractical to process the vitrifiable component by passing it through the gap between electrodes of a non-transferred plasma torch.
The next best alternative is to heat the vitrifiable component in a large crucible or basket with a transferred plasma arc or a gaseous plasma arc jet of a non-transferred plasma torch. The crucible or basket is generally lined with a refractory material. Nevertheless, care must be exercised to avoid the destruction of the refractory lining by erosion from the super high temperatures of the plasma arc jet. This problem has been addressed by rotating the support so that the plasma arc jet is not focused upon the same location of the support for an undue length of time. Unfortunately this method of avoiding erosion of the support leads to excessive heat loss from areas of the melt not in direct contact with the flame at any given time. The problem is further multiplied when the crucible size is increased in an effort to increase the batch size because the plasma jet flame is only a few square inches while the area of a large crucible may be several thousand square inches.
A further problem is introduced by the nature of the melt thus formed. The melt is extremely viscous and may solidify into a glass-like material upon slight cooling. In the solid state the thermal conductivity of the material is very low and further cooling may be extremely slow. Thus, handling and continuous removal of the material is a difficult problem.
Batch processing has been accomplished by pouring the melt into a slag bucket. Not only is this method limited in throughput capability, but also all of the heat content of the melt is lost, rendering the process extremely uneconomical. Moreover, a significant amount if the melt remains in the crucible and does not drain, which further reduces the batch size.